The design of an internal combustion engine requires numerous trade-offs between conflicting design or performance parameters and particularly with respect to camshaft design and thereby valve actuation.
For example, in the design of an engine, a designer may wish to minimize exhaust emissions and provide increased fuel economy without compromising satisfactory engine performance. In the past, the design of such an engine would be limited by such conflicting parameters leading the designer to compromise with the design to achieve a balance between the parameters. As such, designers will often focus on a primary performance goal (such as lower emissions) which may be detrimental to the desired engine performance (such as torque or idle stability). Such compromises are often caused by the designer's failure to incorporate breathability into the engine, as represented by optimal intake of fuel and air and the exhaust of spent gases after combustion.
The breathability of an engine is primarily determined by the physical structure of the camshaft, cam lobes, valve lifters (and the associated push-rods, or rocker arms, if applicable). In particular, the physical shapes or profiles of the cams and their relative orientation with respect to one another determine the timing of the intake and exhaust valve opening, the duration of opening, the valve lift, and the timing of valve closure which, along with the orientation of respective intake and exhaust valves about the camshaft, determine the power map of the cylinder.
As a result of the high-temperature, high-pressure and mechanical speed of the working environment as well as the physical complexity of these components, adjustment of valves during operation of the engine is difficult and accordingly, most engines utilize a fixed cam lobe design wherein the relative parameters of valve operation does not vary with engine speed. As a result, fixed cam lobe engines require trade-offs between the performance parameters of the engine.
More specifically, the function of the camshaft is to open and close valves at the proper time, to fill the cylinders before combustion and to empty them after combustion. The cam lobes are mounted on the camshaft and have a profile, which determines the timing of valve opening, the valve lift, and the duration of opening and the timing of valve closing. The cam followers are in intimate contact with the surfaces of the cams and ride these surfaces in order to impart opening/closing forces to the valves. The opening and closing of valves is thereby timed to the rotation of the camshaft, which in turn is controlled by the crankshaft.
Accordingly, the physical dimensions or shapes of the cams, lifters and the orientation of the cams with respect to one another are parameters, which can be varied in order to obtain desired engine performance.
With respect to the physical dimensions or design of a cam, various terms are generally used to describe the shape of a cam and the physical movements of a valve. For example, the “base circle” of the cam defines the period that the valve is closed, the “clearance ramp” defines the time of transition between closure and measurable valve lifting, the “flank” or “ramp” provides the time for and characteristics of valve opening, the nose defines the time of full valve opening and maximum opening displacement and the “duration” defines the time that the valve is off its seat.
Each of these parameters of a cam cannot be independently controlled during engine operation and therefore require compromises between what the physical dimensions of a cam will allow in relation to the other parameters. For example, duration is a compromise between opening the valves long enough to fill and/or evacuate the cylinders to the loss of dynamic compression by opening the valves too long and increasing lift increases power but is limited by lifter diameter.
With respect to the design of lifters (or tappets), the technology of lifters is variable between engines. Generally, the primary goal of lifter design is to maintain contact between the lifter surface and cam surface while minimizing noise during operation. The two main classes of lifters are solid lifters and hydraulic lifters with each class providing variable contact ends including flat ends, mushrooms and rollers. The use of hydraulic lifters generally reduces valve lash and noise. A flat tappet-cam normally has a slight taper across its surface whereas the corresponding tappet end surface is normally marginally convex in order to compensate for mis-aligned lifter bores.
Roller lifters include a wheel or roller in contact with the cam. Roller lifters allow for highly aggressive ramp profiles and, as a result, require high valve spring tensions to keep the roller in contact with the cam. Roller lifters also reduce frictional losses between the lifter and cam and thereby will increase the overall power or efficiency of the engine.
Mushroom lifters have a bulge at the end and are used to provide more lift per duration.
The relative orientation of the intake and exhaust cams with respect to one another contributes to defining the power map of the engine. Specifically, the lobe separation angle or overlap determines the time during which the intake and exhaust valves are opened simultaneously, wherein a wider lobe separation angle generally improves idle quality, idle vacuum and top-end power whereas a narrower lobe separation angle decreases idle quality but provides better mid-range torque.
Degreeing a cam is also a parameter which can be used to affect engine performance and refers to altering the point where the cam activates the valves in relation to the crankshaft. Specifically, retarding the camshaft, that is, opening a valve later relative to the crankshaft moves the power up the rpm band and can increase horsepower while decreasing lower end torque. In contrast, advancing the camshaft (opening the valves earlier) has the opposite effect.
In order to address some of the problems associated with fixed cam timing, variable cam timing systems have been designed. Generally, such systems provide a cam lobe having a three-dimensional surface and a lifter which is allowed to move axially over the three-dimensional cam surface. Accordingly, the axial position of the camshaft will determine the specific cam profile which controls valve timing.
For example, by diluting the in-cylinder mixture by reducing fuel intake characteristics by providing shorter intake times increases fuel economy but decreases the torque response of the engine. In contrast, by enriching the in-cylinder mixture by increasing fuel intake times by providing more lift and duration leads to an increase in horsepower. A variable valve timing system can accommodate such conflicting objectives by providing different cam profiles depending on the speed of the engine (revolutions per minute) thereby contributing to improvements in the breathability of the engine and increasing the manifold pressure.
In high performance applications, the current state-of-the-art recognizes the single axis roller or wheel based lifter as the optimal performance enhancing device for valve train operation. However, as the desire for higher engine speed has grown, it has been found that wheel based lifters will fail under the higher tension springs utilized in engines configured for higher speeds. Typically, failure occurs in two ways; roller bearing failure in the wheel itself and/or the catastrophic failure of the lifter, both a result of wheel “flat spotting” which produces vibrations in the valve lifter and valve train.
Furthermore, existing wheel-based lifter designs do not provide direct delivery of lubrication to the roller bearing but rather lubrication occurs indirectly which decreases the ability to dissipate heat from the bearing surfaces. Accordingly, bearing life may be reduced as the wheel may be in direct contact with the bearing race with minimal oil film between the two surfaces.
To achieve maximum bearing life in a single axle based system, the designer must balance three parameters given that the wheel diameter is maximized within the confines of the lifter body. These three factors are roller bearing diameter, axle diameter and wheel thickness. Each of these parameters must be varied to minimize the compressive and contact stresses on the bearing surfaces, minimize the stresses in the axle and minimize the deflection of the axle which directly affects the contact stresses within the roller bearings.
While past variable valve timing systems have been disclosed, for example in U.S. Pat. No. 2,969,051, German publication DE 197 55 937, Swiss publication CH 304494 and U.S. Pat. No. 2,307,926, and PCT Publication No. W002/12682, the lifter/cam contacting systems have not experienced widespread implementation or success. The reason for this lack of success is postulated to be a result of failures experienced in the actual implementation of such systems. That is, within the harsh operating conditions of an internal combustion engine, it is speculated that previous variable valve timing systems experience bearing failure within the bearings/races of these systems.